Buried-Contact Organic Field-Effect Transistor: The Way of Alleviating Drawbacks from Interfacial Charge Transfer

Facile charge transfer between source/drain (S/D) electrodes and organic semiconductor (OSC) channel is crucial for high-mobility organic field-effect transistors (OFETs). Herein, a novel OFET geometry is developed by modifying a top-contact bottom-gate device structure, termed a buried-contact OFET, enabling close proximity between the S/D-OSC interface and conducting channel, consequently decreasing the access contact resistance (R_C,acc) and overall contact resistance (R_C). Conventional post-thermal annealing is combined with a burying pressure (pressure-thermal annealing (PTA)). The synergistic effect of thermal and pressure annealings leads to the softened OSC layer enabling metal electrodes to bury inward by applied pressure. This process induces structural transitions from a top-contact to buried-contact configuration, as verified by atomic force microscopy and finite element simulations. Transfer line method and 4-probe measurements revealed that PTA reduces the contact by 1/3 (65 kΩ cm) and the source-to-drain voltage waste due to charge injection from 52% to 31%. Consequently, the field-effect mobility is four times higher than that of a conventional thermally annealed top-contact OFET. The density of deep traps (N_tr) is mainly distributed in the OSC bulk responsible for charge injection. Remarkably, the N_tr decreased 30-fold using PTA, resulting in a shallow sub-threshold region and a threshold voltage close to zero.